KR20050050137A - High-strength steel product excelling in fatigue strength and process for producing the same - Google Patents

Abstract

A high-strength steel product having excellent strength and fatigue strength such that the base metal strength is 1000 MPa or greater and the rotating bending fatigue strength 550 MPa or greater. In particular, a high-strength steel product comprising 0.3 to 0.8 mass% of C, 0.01 to 0.9 mass% of Si, 0.01 to 2.0 mass% of Mn and the balance of Fe and unavoidable impurities, wherein the steel structure is a ferrite/cementite structure of 7 mum or less grain diameter or a ferrite/cementite/pearlite structure of 7 mum or less grain diameter. Further, the surface layer part thereof after high-frequency hardening has a martensite structure of 12 mum or less old austenite grain diameter. Alternatively, the surface layer part thereof after nitriding has a fine ferrite structure of 10 mum or less grain diameter.

Description

High strength steel with excellent fatigue strength and manufacturing method {HIGH-STRENGTH STEEL PRODUCT EXCELLING IN FATIGUE STRENGTH AND PROCESS FOR PRODUCING THE SAME}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength steel having excellent fatigue strength and a method for producing the same, which are preferably applied to automobile parts using crude steel, for example, constant velocity joints, drive shafts, crank shafts, connecting rods and hubs.

Products such as connecting rods and hubs have been manufactured by hot forging or rolling and then cutting. Products such as constant velocity joints, drive shafts, crankshafts and hubs have been produced by hot forging or forging after annealing or spherical annealing to increase machinability, and then by partly or wholly high frequency quenching or nitriding. For products for this purpose, high strength and high fatigue life are required to reduce the weight of the vehicle body.

Conventionally, in order to improve fatigue strength, it is known that reducing the maximum diameter of inclusions and reducing the number of inclusions is most effective.

For example, Patent Document 1 discloses a method of improving fatigue strength by appropriately adjusting each component such as Al, N, Ti, Zr and S by setting the maximum diameter of the sulfide to 10 µm or less and the cleanness to 0.05% or more. Is proposed. However, particularly in high strength materials, when cyclic stress is applied, grain boundary breakage tends to occur, and thus the target fatigue strength cannot be obtained.

Further, for example, Patent Document 2 discloses fatigue characteristics by suppressing 20 or less oxides and sulfides in a linear or bar-shaped rolled steel in a unit area of 100 mm 2 parallel to the axis center and distant from the axis center by 1/4. And a method for improving the rolling fatigue characteristic have been proposed. However, in this method, the maximum fatigue strength is only about 770 MPa, and cannot meet the recent demand for bending fatigue strength.

Patent Document 1 Japanese Patent Application Laid-Open No. 11-302778

Patent Document 2 Japanese Patent Application Laid-Open No. 11-1749

(Initiation of invention)

The present invention was developed in view of the above-mentioned phenomenon, and by controlling the structure appropriately with the composition adjustment, a high-strength steel having excellent strength and fatigue strength having a strength of 1000 MPa or more and a rotation bending fatigue strength of 550 MPa or more is an advantageous manufacturing method thereof. It is intended to propose together with.

In addition, the present invention advantageously produces a high-strength steel having excellent strength and fatigue strength, in which the base material strength is 1000 MPa or more and the rotation bending fatigue strength after high frequency quenching or nitriding is 800 MPa or more by appropriately controlling the base material structure and the surface layer structure. It is intended to propose along with the method.

The inventors earned the following knowledge as a result of earnest research to achieve the above object.

(1) If the grain size of the steel is finer, the strength and the fatigue strength are improved together. However, if the grain size is simply fined, the fatigue strength targeted by the present invention is not obtained.

(2) When the component is adjusted so that not only the fine grain ferrite but also fine grain cementite is produced, the fatigue strength is effectively improved. Moreover, since this fine dispersion cementite has the effect | action which enlarges uniform elongation, the workability of a material improves.

(3) In order to make the steel structure into a structure having fine ferrite and fine cementite, it is effective to perform deformation 1.0 or more processing at a temperature range of 550 to 700 ° C in addition to the steel component adjustment.

(4) Finer grain size improves both strength and fatigue strength, but simply fine grain size results in coarsening of crystal grains by high frequency quenching. Is not obtained.

(5) If the components are adjusted so that not only fine ferrite but also fine cementite are produced in the steel structure, the finely dispersed cementite and the base material ferrite grain system act as nuclei of austenitization at high frequency heating, and austenite from a plurality of nuclei Since marring occurs, the final austenite grain size of martensite finally obtained is also reduced. As a result, the strength and fatigue strength are significantly improved even after high frequency quenching.

(6) In high frequency quenching, the relatively low temperature has a significant improvement effect.

(7) If the grain size of the steel is finer, the strength and the fatigue strength are improved together. However, in the case where the nitriding treatment is performed on the surface layer portion, the grain size is coarsened by subsequent nitriding treatment simply by making the grain size finer. Therefore, the fatigue strength aimed at by this invention is not obtained.

(8) When the component is adjusted so that not only fine ferrite but also fine cementite is produced in the steel structure, the finely dispersed cementite plays a role of pinning during nitriding, thereby suppressing ferrite grain growth. Therefore, the ferrite particle diameter of the surface layer part finally obtained becomes fine. As a result, even after nitriding treatment, the strength and fatigue strength are significantly improved.

(The best mode for carrying out the invention)

That is, the summary structure of this invention is as follows.

1.C: 0.3-0.8 mass%,

Si: 0.01-0.9 mass% and

Mn: 0.01-2.0 mass%

And the balance is composed of Fe and unavoidable impurities, and the structure is a ferrite and cementite structure having a particle diameter of 7 μm or less, or a ferrite, cementite and pearlite structure having a particle size of 7 μm or less, and has excellent fatigue strength. Steel.

2. In 1 steel, further,

Mo: 0.05-0.6 mass%

A high strength steel having excellent fatigue strength, characterized by being a composition containing a.

3. For 2 steels, additionally

Al: 0.015-0.06 mass%,

Ti: 0.005-0.030 mass%,

Ni: 1.0 mass% or less,

Cr: 1.0 mass% or less,

V: 0.1 mass% or less,

Cu: 1.0 mass% or less,

Nb: 0.05 mass% or less,

Ca: 0.008 mass% or less and

B: 0.004 mass% or less

A high strength steel having excellent fatigue strength, characterized by being a composition containing one or two or more selected from among them.

4. The high strength steel having excellent fatigue strength according to 1, 2 or 3, wherein the cementite has a structure fraction of 4 vol% or more.

5. Steel 2, wherein the surface layer after high-frequency quenching is a martensitic structure having an austenite grain size of 12 µm or less.

6. For 5 steels, additionally

Al: 0.015-0.06 mass%,

Ti: 0.005-0.030 mass%,

Ni: 1.0 mass% or less,

Cr: 1.0 mass% or less,

V: 0.1 mass% or less,

Cu: 1.0 mass% or less,

Nb: 0.05 mass% or less,

Ca: 0.008 mass% or less and

B: 0.004 mass% or less

A high strength steel having excellent fatigue strength, characterized by being a composition containing one or two or more selected from among them.

7. For 2 steels, in addition,

The ferrite grain size of the surface layer part after nitriding is 10 micrometers or less, The high strength steel provided with the hardening layer by nitriding in the surface layer part of steel materials excellent in fatigue strength.

8. For the steel of 7, furthermore,

Al: 0.015-0.06 mass%,

Ti: 0.005-0.030 mass%,

Ni: 1.0 mass% or less,

Cr: 1.0 mass% or less,

V: 0.1 mass% or less,

Cu: 1.0 mass% or less,

Nb: 0.05 mass% or less,

Ca: 0.008 mass% or less, and

B: 0.004 mass% or less

A high strength steel having excellent fatigue strength, characterized by being a composition containing one or two or more selected from among them.

9. The high-strength steel having excellent fatigue strength according to 7 or 8, wherein the structure fraction of cementite in the base metal structure is 4 vol% or more.

10. C: 0.3-0.8 mass%,

Si: 0.01-0.9 mass% and

Mn: 0.01-2.0 mass%

Wherein the remainder is subjected to processing with a strain of 1.0 or more at a temperature in the range of 550 to 700 ° C. in which the remainder is composed of Fe and unavoidable impurities.

11. The steel material of 10 is further

Mo: 0.05-0.6 mass%

A method for producing a high strength steel having excellent fatigue strength, characterized by containing a.

12. The steel material of 11 is further

Al: 0.015-0.06 mass%,

Ti: 0.005-0.030 mass%,

Ni: 1.0 mass% or less,

Cr: 1.0 mass% or less,

V: 0.1 mass% or less,

Cu: 1.0 mass% or less,

Nb: 0.05 mass% or less,

Ca: 0.008 mass% or less and

B: 0.004 mass% or less

A method for producing a high strength steel having excellent fatigue strength, characterized in that the composition contains one or two or more selected from among them.

13. With 11,

550 ~ 700 ℃ for steel

A method for producing a high strength steel having excellent fatigue strength, wherein the deformation is performed at a temperature range of 1.0 or more, followed by high frequency quenching.

14. The material of 13, further comprising:

Al: 0.015-0.06 mass%,

Ti: 0.005-0.030 mass%,

Ni: 1.0 mass% or less,

Cr: 1.0 mass% or less,

V: 0.1 mass% or less,

Cu: 1.0 mass% or less,

Nb: 0.05 mass% or less,

Ca: 0.008 mass% or less and

B: 0.004 mass% or less

A method for producing a high strength steel having excellent fatigue strength, characterized in that the composition contains one or two or more selected from among them.

15. The method of 11,

A method for producing a high strength steel having excellent fatigue strength, wherein the steel material is subjected to processing having a strain of 1.0 or more at a temperature range of 550 to 700 ° C., followed by nitriding treatment on the surface layer portion.

16. The steel material according to 15, further

Al: 0.015-0.06 mass%,

Ti: 0.005-0.030 mass%,

Ni: 1.0 mass% or less,

Cr: 1.0 mass% or less,

V: 0.1 mass% or less,

Cu: 1.0 mass% or less,

Nb: 0.05 mass% or less,

Ca: 0.008 mass% or less and

B: 0.004 mass% or less

A method for producing a high strength steel having excellent fatigue strength, characterized in that the composition contains one or two or more selected from among them.

Hereinafter, the present invention will be described in detail. First, the reason which limited the component composition of steel materials to said range in this invention is demonstrated.

C: 0.3-0.8 mass%

C is an element necessary for increasing the strength of the base metal and securing the required amount of cementite. Here, if the C content is not 0.3% by mass, the above effects are not obtained. On the other hand, if the C content is more than 0.8% by mass, the machinability, fatigue strength, and forging are lowered. Therefore, the amount of C is 0.3 to 0.8% by mass. It was limited to the range.

Si: 0.01-0.9 mass%

Si not only acts as a deoxidizer but also contributes effectively to the improvement of strength, but if the content is not 0.01% by mass, the effect of addition is inferior. On the other hand, if the content exceeds 0.9% by mass, the amount of Si is reduced. Was limited to the range of 0.01-0.9 mass%.

Mn: 0.01-2.0 mass%

Mn effectively contributes not only to the improvement of strength but also to the improvement of fatigue strength, but if the content does not reach 0.01% by mass, the effect of addition decreases. On the other hand, if the content exceeds 2.0% by mass, Mn deteriorates machinability and forging. It limited to the range of -2.0 mass%.

As mentioned above, although the basic component was demonstrated, in this invention, the element described below can be contained suitably.

Mo: 0.05-0.6 mass%

Mo is an element that is useful for suppressing the growth of ferrite particles, but at least 0.05 mass% is required for this purpose, but if it is added in excess of 0.6 mass%, the amount of Mo is 0.05 to 0.6 mass. It was limited to the range of%.

Al: 0.015-0.06 mass%

Al acts as a deoxidizer for steel. However, when the content is not 0.015% by mass, the addition effect is inferior, while when it exceeds 0.06% by mass, the machinability and the fatigue strength are reduced, so the Al content is limited to the range of 0.015 to 0.06% by mass.

Ti: 0.005-0.030 mass%

Ti is an element that is useful for miniaturizing crystal grains by the pinning effect of TiN, and at least 0.005 mass% of addition is required to obtain this effect, but addition of more than 0.030 mass% causes a decrease in fatigue strength. , Ti amount was limited to the range of 0.005 to 0.030 mass%.

Ni: 1.0 mass% or less

Ni is effective for preventing the increase in strength and cracking when Cu is added. However, since Ni is more likely to cause quenched cracking when added in excess of 1.0% by mass, Ni is limited to 1.0% by mass or less.

Cr: 1.0 mass% or less

Cr is effective in increasing strength, but addition of more than 1.0% by mass stabilizes the carbides to promote the formation of residual carbides, lower the grain boundary strength, and also reduce the fatigue strength, so it is limited to 1.0% by mass or less. .

V: 0.1 mass% or less

V is a useful element which becomes a carbide and precipitates, and exhibits the structure micronizing effect by pinning, but since the effect is saturated even if it exceeds 0.1 mass%, it was limited to 0.1 mass% or less.

Cu: 1.0 mass% or less

Cu is a useful element that improves strength by solid solution strengthening and precipitation strengthening, and effectively contributes to the improvement of hardenability. However, if the content exceeds 1.0% by mass, Cu is prone to cracking during hot working, making manufacturing difficult. It was limited to 1.0 mass% or less.

Nb: 0.05 mass% or less

Although Nb has an effect of pinning ferrite particles by precipitation, even if it exceeds 0.05 mass%, since the effect is saturated, it was limited to 0.05 mass% or less.

Ca: 0.008 mass% or less

Ca is a useful element for spherical inclusions to improve fatigue characteristics. However, Ca is limited to 0.008% by mass or less because the inclusions tend to coarsen and conversely deteriorate fatigue characteristics.

B: 0.004 mass% or less

B is a useful element that not only improves the fatigue characteristics by strengthening the grain boundary but also improves the strength, but the effect is saturated even if it is added exceeding 0.004% by mass, so it is limited to 0.004% by mass or less.

Although the suitable component composition was demonstrated above, in this invention, just limiting a component composition to the said range is insufficient, and adjustment of a steel structure is also important as mentioned below.

The structure is a ferrite and cementite structure having a particle size of 7 μm or less, or the ferrite, cementite and pearlite structure having a particle size of 7 μm or less.

If the structure is not 7 µm or less of ferrite and cementite structure, or 7 µm or less of ferrite, cementite and pearlite structure, the strength ≧ 1000 MPa, which is a target in the present invention, is not obtained. Therefore, the ferrite particle diameter was limited to 7 µm or less. More preferably, it is 5 micrometers or less.

If the base material structure, that is, the structure before high frequency quenching (corresponding to parts other than the surface layer quenching structure after high frequency quenching) is not a ferrite and cementite structure having a particle diameter of 7 μm or less, or a ferrite, cementite and pearlite structure having a particle size of 7 μm or less, The base material strength of 1000 Mpa or more which is the objective in this invention is not obtained. In addition, if the ferrite grain size is not 7 µm or less, when the high frequency quenching is applied thereafter, the old austenite grain size of the high frequency quenching portion does not become 12 µm or less, and thus the fatigue strength is not improved. Therefore, the ferrite particle diameter of the base material was limited to 7 micrometers or less. More preferably, it is 5 micrometers or less.

If the base material structure, that is, the structure before nitriding (corresponding to portions other than the surface layer nitride layer after nitriding), is not a ferrite and cementite structure having a particle diameter of 7 μm or less, or a ferrite, cementite, and pearlite structure having a particle size of 7 μm or less, The base material strength of 1000 MPa or more which is the objective in this invention is not obtained. If the ferrite grain size is not 7 µm or less, and the nitride treatment is subsequently performed, the ferrite grain size of the nitride layer does not become 10 µm or less. Therefore, the ferrite particle diameter of the base material was limited to 7 micrometers or less. More preferably, it is 5 micrometers or less.

Moreover, when a ferrite particle diameter becomes 2 micrometers or less, a pearlite structure may lose | disappear and it may become a ferrite-cementite structure, but this does not inhibit this invention.

In addition, it is preferable that the amount (tissue fraction) of precipitated cementite is 4% or more by volume fraction (vol%). Cementite not only contributes to the improvement of the fatigue strength, but also has the effect of increasing the uniform elongation by increasing the fine deposition in large quantities to improve the workability of the material. Here, it is preferable that the size of the precipitated cementite is about 1 micrometer or less. More preferably, it is 0.5 micrometer or less. In addition, the amount of pearlite deposited is preferably about 20 vol% or less. This pearlite does not need to be precipitated at all as mentioned above. In addition, remaining structures other than cementite and pearlite are ferrite. It is preferable to make this ferrite amount into 40 volume% or more from a viewpoint of workability securing. In addition, the above-described ferrite and cementite structures or ferrite, cementite and pearlite structures can be preferably obtained by performing a process in which deformation is 1.0 or more at a temperature range of 550 to 700 ° C. in a warm forging step of a steel manufacturing process. have.

Martensite structure whose surface layer part after high frequency quenching has a former austenite particle diameter of 12 micrometers or less

If the former austenite grain size is not 12 µm or less, high bending fatigue strength of 800 MPa or more, which is a target in the present invention, cannot be obtained. Therefore, the former austenite particle diameter in the structure after high frequency quenching was limited to 12 micrometers or less. Preferably it is 5 micrometers or less.

The above-described high-frequency quenched structure can be obtained by subjecting the base material structure to ferrite and cementite structure having a particle diameter of 7 μm or less, or ferrite, cementite and pearlite structure having a particle size of 7 μm or less, and then performing high frequency quenching under the conditions described below. have.

Ferrite particle size of surface layer part after nitriding is 10 µm or less

If the surface layer portion after the nitriding treatment, that is, the ferrite grain size of the nitride layer is not 10 µm or less, high bending fatigue strength of 800 MPa or more, which is a target in the present invention, cannot be obtained. Therefore, the ferrite particle diameter in the surface layer structure after nitriding treatment was limited to 10 micrometers or less. Preferably it is 5 micrometers or less.

In addition, the surface layer structure after the above-mentioned nitriding treatment is performed by nitriding the base metal structure with ferrite and cementite structure having a particle diameter of 7 μm or less, or ferrite, cementite and pearlite structure having a particle size of 7 μm or less, and then subjecting to nitriding under the conditions described below. You can get it.

Next, the manufacturing conditions of the steel of the present invention will be described.

First, steel rods adjusted to a predetermined component composition are subjected to wire rod rolling, followed by warm forging. It is based on warm forged steel. Warm forged steel is subjected to finishing such as cutting to commercialize. Alternatively, cold forged steels may be subjected to cold quenching if necessary, followed by high frequency quenching to obtain products. Or after performing a process, such as a cutting, as needed for a steel forging which is warm forging, it nitrides it and manufactures it.

In the warm forging step, in order to make the ferrite grain size 7 µm or less, it is advantageous to perform processing in which the strain is 1.0 or more in the temperature range of 550 to 700 ° C. In this case, when the processing temperature is less than 550 ° C., the tissue remains as it is and does not become micronized. On the other hand, when the processing temperature exceeds 700 ° C, the grain size exceeds 7 µm and is not refined again. In addition, when the processing amount is less than 1.0 in deformation, processing is insufficient and small angle boundaries occupy most of them, so that strength and fatigue properties are not improved.

After making it into the said base material structure, high frequency quenching is performed and hardening a surface layer part. The high frequency quenching condition at this time can employ | adopt heating temperature: 800-1000 degreeC, and frequency: 0.3-400 kHz. If the heating temperature does not reach 800 ° C, austenitization is insufficient. On the other hand, if the heating temperature exceeds 1000 ° C, the austenite grain size becomes coarse. On the other hand, if the frequency does not become 0.3 kHz, rapid and sufficient temperature rise is not obtained. On the other hand, if it exceeds 400 kHz, the quenching depth becomes shallow, and the bending fatigue strength does not improve.

After making it into the said base material structure, it nitrate-processes and hardens a surface layer part and improves abrasion resistance. The nitriding treatment condition at this time is maintained in a nitriding atmosphere for 1 to 100 hours in a temperature range of 500 to 650 ° C. In this nitriding treatment, the raw material of nitrogen may be a gas or a liquid.

If the nitriding temperature does not reach 500 ° C, nitrogen is difficult to enter into the river, and sufficient nitriding cannot be expected. On the other hand, when it exceeds 650 degreeC, it is difficult to suppress grain growth of a base material, and a ferrite particle coarsens. In addition, if the nitriding time is less than 1 h, since nitrogen does not sufficiently enter the river, the effect of nitriding is small, and the effect is saturated even if the nitriding treatment exceeds 100 h.

Example 1

After rod-rolling the steel material used as the component composition shown in Table 1, it forged warmly on the conditions shown in Table 2, and obtained the product of 60x60x120 mm. Tensile test pieces, rotational bending fatigue test pieces and machinability test pieces were taken from this product. Table 2 shows the results of investigations on the grain size of ferrite, the amount of cementite and the amount of pearlite, the tensile strength, the bending bending fatigue strength and the machinability of the product. In addition, the deformation amount at the time of warm forging was computed by the finite element analysis method which made the friction coefficient of a forging surface 0.3. In addition, machinability evaluated the case where tool life in outer turning test was equivalent or more than SC material of JIS G5101 as (circle), and the case where it fell from SC material as x.

As can be seen from Table 2, according to the present invention, both the ferrite and cementite or the ferrite and cementite and pearlite tissues having a particle diameter of 7 µm or less can obtain not only excellent strength of ≧ 1000 MPa. Excellent fatigue strength of rotation bending fatigue strength ≥550MPa was obtained.

On the other hand, in the comparative example of No. 6 with a small deformation amount at the time of forging, ferrite grains are not refined and rotational bending fatigue strength is low. In addition, in the comparative example of No. 7 having a low forging temperature, the structure became a processed structure, while in the comparative example of No. 8 having a high forging temperature, the ferrite particles did not become fine, and therefore the rotation bending fatigue strength was low.

In addition, in the comparative example of No. 13 in which the amount of Mo was excessive, machinability fell. And in the comparative example of No. 14 which lacks C amount, intensity | strength was insufficient, but in the comparative example of No. 15 in which C is excess, the machinability fell.

Example 2

After rod-rolling the steel material used as the component composition shown in Table 3, it warm-forged on the conditions shown in Table 4, and obtained the base material of 60x60x120 mm. Tensile test pieces, rotational bending fatigue test pieces, and machinability test pieces were taken from the base material. Subsequently, high frequency quenching was performed on the rotation bending fatigue test piece on the conditions of a heating temperature of 900 degreeC and a frequency of 12 kHz. Table 4 shows the results of the ferrite crystal grain size, cementite amount, pearlite amount, tensile strength and machinability of the base material, and the austenite grain size of the quenched structure after high frequency quenching and the rotational bending fatigue strength of the test piece after high frequency quenching. . In addition, the deformation amount at the time of warm forging was computed by the finite element analysis method which made the friction coefficient of a forging surface 0.3. Moreover, machinability evaluated the case where tool life in outer turning test was equivalent to or more than normal SC material, and the case where it fell below SC material as x.

As can be seen from Table 4, according to the present invention, all of the examples of the base material structure having a ferrite and cementite structure having a particle diameter of 7 μm or less, or a ferrite, cementite, and pearlite structure having a particle size of 7 μm or less have a base material strength of 1000 MPa or more. Not only the excellent strength was obtained, but the surface layer structure after high frequency quenching also became a fine martensite structure having an austenite grain size of 12 µm or less, and an excellent fatigue strength with a rotation bending fatigue strength of 800 MPa or more was obtained.

On the other hand, when the ferrite particle diameter of the base material was more than 7 µm, the base material strength was insufficient, and the particle size of the austenite after high frequency quenching was also coarsened, and the rotation bending fatigue strength was also insufficient.

In particular, in the comparative example of No. 7 having a low forging temperature, the structure became a processed structure, while in the comparative example of No. 8 having a high forging temperature, the ferrite particles did not refine. In addition, even if the coarse ferrite structure is subjected to high frequency quenching, the obtained austenite grain size of martensite does not become 12 µm or less.

In addition, in the comparative example of No. 12 which did not add Mo, the base material ferrite particle was refine | miniaturized but the old austenite particle size after high frequency quenching became coarse. On the other hand, in the comparative example of No. 13 in which Mo amount is excessive, machinability fell.

In addition, in the comparative example of No. 14 in which the amount of C was insufficient, it was not quenched, while in the comparative example of No. 15 in which C was excess, the machinability was lowered.

Example 3

After rod-rolling the steel material used as the component composition shown in Table 5, it forged warmly on the conditions shown in Table 6, and obtained the base material of 60x60x120 mm. Tensile test pieces, rotational bending fatigue test pieces, and machinability test pieces were taken from the base material. Next, about the rotation bending fatigue test piece, it nitrided on the conditions shown in Table 6. Table 6 lists the ferrite grain size, cementite amount, pearlite amount, tensile strength, and machinability and the ferrite grain size and rotational bending fatigue strength of the surface layer after nitriding. In addition, the deformation amount at the time of warm forging was computed by the finite element analysis method which made the friction coefficient of a forging surface 0.3. Moreover, machinability evaluated the case where tool life in outer turning test was equivalent to or more than normal SC material, and the case where it fell below SC material as x.

As can be seen from Table 6, according to the present invention, all of the examples of the base material structure having a ferrite and cementite structure having a particle diameter of 7 μm or less or a ferrite, cementite, and pearlite structure having a particle size of 7 μm or less have a base material strength of 1000 MPa or more. In addition to obtaining excellent strength, the surface layer structure after nitriding also became a fine structure having a ferrite grain size of 10 µm or less, and an excellent fatigue strength of rotation bending fatigue strength ≥ 800 MPa was obtained. Moreover, the machinability was also excellent.

On the other hand, when the ferrite particle size of the base material exceeded 7 micrometers, the ferrite particle size after nitriding was also coarsened, and rotational bending fatigue strength was inadequate.

In particular, in the comparative example of No. 6 having a low forging temperature, the structure became a processed structure, while in the comparative example of No. 7 having a high forging temperature and No. 8 having a small deformation amount during forging, the ferrite particles were not refined. Further, even when such a coarse ferrite structure was subjected to nitriding treatment, the ferrite grain size of the nitride region did not become 10 µm or less.

Moreover, in the comparative example of No. 13 which did not add Mo, the base material ferrite particle was refine | miniaturized, but the ferrite particle size after nitriding process became large and rotation bending fatigue strength was inadequate. In the comparative example of No. 1 lacking the amount of C, the ferrite grain size after nitriding was coarse, and the base material strength and the rotation bending fatigue strength were insufficient. On the other hand, in the comparative example of No. 4 in which C is excess, the machinability was reduced. Moreover, in the comparative example of No. 9 which did not nitride, sufficient rotation bending fatigue strength was not obtained.

According to the present invention, it is possible to stably obtain a high-strength steel having excellent strength and fatigue strength, in which the base material strength is 1000 MPa or more, the rotation bending fatigue strength is 550 MPa or more, or the rotation bending fatigue strength is 800 MPa or more.

Claims (16)

C: 0.3-0.8 mass%, Si: 0.01-0.9 mass% and Mn: 0.01-2.0 mass% And the balance is composed of Fe and unavoidable impurities, and the structure is a ferrite and cementite structure having a particle diameter of 7 μm or less, or a ferrite, cementite and pearlite structure having a particle size of 7 μm or less, and has excellent fatigue strength. Steel. The method of claim 1 further comprising: Mo: 0.05-0.6 mass% A high strength steel having excellent fatigue strength, characterized by being a composition containing a. The method of claim 2, further Al: 0.015-0.06 mass%, Ti: 0.005-0.030 mass%, Ni: 1.0 mass% or less, Cr: 1.0 mass% or less, V: 0.1 mass% or less, Cu: 1.0 mass% or less, Nb: 0.05 mass% or less, Ca: 0.008 mass% or less and B: 0.004 mass% or less A high strength steel having excellent fatigue strength, characterized by being a composition containing one or two or more selected from among them. The high-strength steel having excellent fatigue strength according to claim 1, 2 or 3, wherein the cementite has a structure fraction of 4 vol% or more. 3. The high strength steel having excellent fatigue strength according to claim 2, wherein the surface layer portion after high frequency quenching is a martensite structure having a prior austenite particle diameter of 12 µm or less. The method of claim 5, further comprising Al: 0.015-0.06 mass%, Ti: 0.005-0.030 mass%, Ni: 1.0 mass% or less, Cr: 1.0 mass% or less, V: 0.1 mass% or less, Cu: 1.0 mass% or less, Nb: 0.05 mass% or less, Ca: 0.008 mass% or less and B: 0.004 mass% or less A high strength steel having excellent fatigue strength, characterized by being a composition containing one or two or more selected from among them. The method of claim 2, further comprising: The ferrite grain size of the surface layer part after nitriding is 10 micrometers or less, The high strength steel provided with the hardened layer by nitriding in the surface layer part of steel materials excellent in fatigue strength. The method of claim 7, further comprising: Al: 0.015-0.06 mass%, Ti: 0.005-0.030 mass%, Ni: 1.0 mass% or less, Cr: 1.0 mass% or less, V: 0.1 mass% or less, Cu: 1.0 mass% or less, Nb: 0.05 mass% or less, Ca: 0.008 mass% or less, and B: 0.004 mass% or less A high strength steel having excellent fatigue strength, characterized by being a composition containing one or two or more selected from among them. The high-strength steel having excellent fatigue strength according to claim 7 or 8, wherein the structure fraction of cementite in the base metal structure is 4 vol% or more. C: 0.3-0.8 mass%, Si: 0.01-0.9 mass% and Mn: 0.01-2.0 mass% Wherein the remainder is subjected to processing with a strain of 1.0 or more at a temperature in the range of 550 to 700 ° C. in which the remainder is composed of Fe and unavoidable impurities. The steel material according to claim 10, further comprising Mo: 0.05-0.6 mass% A method for producing a high strength steel having excellent fatigue strength, characterized by containing a. The steel material according to claim 11, wherein the steel material is further Al: 0.015-0.06 mass%, Ti: 0.005-0.030 mass%, Ni: 1.0 mass% or less, Cr: 1.0 mass% or less, V: 0.1 mass% or less, Cu: 1.0 mass% or less, Nb: 0.05 mass% or less, Ca: 0.008 mass% or less and B: 0.004 mass% or less A method for producing a high strength steel having excellent fatigue strength, characterized in that the composition contains one or two or more selected from among them. The method of claim 11, 550 ~ 700 ℃ for steel A method for producing a high strength steel having excellent fatigue strength, wherein the deformation is performed at a temperature range of 1.0 or more, followed by high frequency quenching. The method of claim 13, wherein the steel material, Al: 0.015-0.06 mass%, Ti: 0.005-0.030 mass%, Ni: 1.0 mass% or less, Cr: 1.0 mass% or less, V: 0.1 mass% or less, Cu: 1.0 mass% or less, Nb: 0.05 mass% or less, Ca: 0.008 mass% or less and B: 0.004 mass% or less A method for producing a high strength steel having excellent fatigue strength, characterized in that the composition contains one or two or more selected from among them. The method of claim 11, A method for producing a high strength steel having excellent fatigue strength, wherein the steel material is subjected to processing having a strain of 1.0 or more at a temperature range of 550 to 700 ° C., followed by nitriding treatment on the surface layer portion. The steel material according to claim 15, further comprising: Al: 0.015-0.06 mass%, Ti: 0.005-0.030 mass%, Ni: 1.0 mass% or less, Cr: 1.0 mass% or less, V: 0.1 mass% or less, Cu: 1.0 mass% or less, Nb: 0.05 mass% or less, Ca: 0.008 mass% or less and B: 0.004 mass% or less A method for producing a high strength steel having excellent fatigue strength, characterized in that the composition contains one or two or more selected from among them.